Holographic interferometry is a known testing technique for nondestructively and economically evaluating various aspects of the structural integrity of pneumatic tires. This technique involves the comparison of two interference patterns which result when a beam of coherent light reflects off the tire and is recorded together with an unreflected portion of the same beam. A first interference pattern is set up and recorded under one set of conditions, a stress (such as environmental pressure) is placed on the tire, and then a second interference pattern is set up and recorded (preferably directly over the first interference pattern). The resulting composite pattern contains a pattern of "fringe lines", or contours of equal displacement. By measurement of the distance between fringe lines tire inner ply separation as minute as a fraction of the wavelength of the coherent light may be detected. Unfortunately, in many instances perturbations other than ply separations result in disturbances in the fringe pattern, making determination of an unacceptable tire on the basis of fringe line separation occasionally insufficiently reliable.
Areas of undercured elastomeric material in the vicinity of a pneumatic tire bead, commonly known as a "blown bead" condition, has been long recognized as a common point of premature failure in pneumatic tires, particularly so when applied in very demanding environments such as aircraft landing gear. Historically the likelihood of such a failure was nondestructively detected, if at all, by a human operator who manually examined the bead region and, entirely by touch, made the determination. Although many operators have developed an uncanny accuracy in making these inspections, such a procedure is unreliable as a result of its total subjectivity and dependence upon the operator's skill.
I have found a method employing holographic interferometry which discloses the existence of a blown bead in a pneumatic tire. This determination is made without measurement and independent of the distance between fringe lines in the holographic interference pattern.